The recent meeting in the Vatican of the "custodian of the holy places," King Abdullah of Saudi Arabia, and Pope Benedict XVI was a seminal event, particularly as it comes at a time when radical Muslims are decrying the role of "crusaders" in Middle East politics. It was also the clearest sign yet of a rising "holy alliance" among the world's conservative leaders. For the key audience for this meeting of Muslim king and Roman Catholic pontiff was not their followers, but another conservative leader, President George Bush.

The first "holy alliance" was a creation of Austria's Prince Metternich following the Napoleonic wars. It was an attempt to preserve the peace (and the security) of the relatively weak Austrian empire, through a coalition of victors upholding shared values.

Metternich's "holy alliance" was the one original political idea to emerge from Napoleon's defeat. Behind its exalted name lay an innovation of great diplomatic significance: the introduction of an element of calculated moral restraint into international relations. The vested interests that the alliance members - Austria, Prussia and Russia - had in the survival of their domestic institutions led each to avoid conflicts that, in the past, they would have pursued as a matter of course.

Metternich's system worked through much of the 19th century, because it protected a genuine balance of power between countries that shared common values. But what "common values" do king, Pope, and American president share?

That such a meeting took place is a sign that both Pope and king believed that there was something to achieve. Indeed, Abdullah, who believes himself to be the paramount leader of the Muslim world, is the first Saudi king to initiate a meeting with a leader of the Christian faith.

The two men met, seemingly as equals, not to express the common values of conservatism, but to confirm and promote their authority to uphold traditional values. Both agreed that reform must be slow, cautious and that it must never undermine established institutions, especially religion and the patriarchal family.

Abdullah sought the meeting because he believes that the world since 2001 has divided the fraternity of conservatives. Until then, he and Bush shared a common worldview, emphasising the importance of religion, the traditional family (as both countries understood it), social discipline, and the state's role in supporting these institutions.

But Bush, following the terrorist attacks of 2001, turned his back on conservatism. He sought to radically reinvent the Middle East, not only by overthrowing the Taliban regime in Afghanistan and Saddam Hussein's regime in Iraq, but also through his strident calls for democratisation.

American radicalism, however, resulted in increasing tensions between religious communities in the Middle East, and the rise of radical Iran with its bid for regional hegemony. Since 2001, Christian minorities have been targeted throughout the region, including even Lebanon's Maronite Christian community. And in Iraq, Sunni Muslims feel under siege from the country's majority and now ruling Shia.

Here is where the holy alliance concept of moral restraint comes in. Abdullah, perhaps earlier than most others, understood that some code of restraint was needed if the entire region was not to descend into a war of all against all.

Moreover, Abdullah understands that his wobbly regime will only be able to withstand the radical gales that are now blowing if it can forge the type of stability-seeking alliance that Metternich built. The king, like Metternich and the Austrian kaisers, understands De Tocqueville's dictum that the most dangerous moment for a regime is when it starts to reform. Having begun, ever so carefully, to politically open his country, the king knows that he needs regional peace and a lowering of Islamic holy rage.

The problem is that Abdullah cannot rely on his domestic conservative allies to give him the time that the kingdom needs. The Wahhabi religious establishment, the Saudi state's hidden co-rulers, could very well obstruct Abdullah's attempts at regional religious reconciliation. Members of the religious police remain adamant that the country's Christian guests must continue to live according to strict Wahhabi rules of behaviour. While the Wahhabis could yet be bribed again with oil money, the sect's power-hungry judges eagerly await the next beheading, stoning, or lashing in Riyadh's public square.

Thus, uniting the forces of status quo conservatism, even if some of those conservatives are Christian, is the only viable diplomatic strategy open to Saudi Arabia. For conservative rulers usually fall when they fail to grasp their own vulnerability, especially when the revolutionary challenge is cloaked in conservative garb. After all, few political systems can defend against those, like Saudi Arabia's Islamic radicals, who claim that they can preserve the system and its religious values more effectively than the current rulers.

Only an alliance of conservative leaders and powers (including a retreat by America from diplomatic radicalism), Abdullah believes, can restore some stability to the Middle East.